1. Field of the Invention
This invention relates generally to television synchronizing signal generators, and more particularly to a selectable rate sync generator system for use with a television test pattern generator.
2. Description of the Prior Art
Television test pattern generators are used to generate a variety of video test patterns which may be transmitted in the case of broadcast television to permit adjustment of a television receiver, or which may be used in the manufacture and service of television receivers and/or transmitting equipment. Typical test pattern generator systems are shown, for example, in the U.S. Pat. Nos. 4,149,178; 4,093,960; 3,917,902; 3,879,749 and 3,019,289.
Conventional television test pattern generators require composite, horizontal and vertical sync, composite, horizontal and vertical blanking, and horizontal and vertical drive inputs, and provide a video signal output. H drive is a pulse train in which a pulse starts at the end of every television scanning line, its function being to mark the time interval between the end of one line and the start of another line. The H drive pulses may be used to control the horizontal deflection "flyback" and other signal processing. The H drive pulse train is continuous and uninterrupted even during the vertical synchronizing interval, and the H drive pulses are useful for synchronizing an oscilloscope when inspecting the television wave form at the horizontal line rate. The V drive is a pulse train in which a pulse starts at the end of each television field, its function being to mark the time interval between the end of one television field and the next. The V drive pulses may be used to control the vertical deflection "flyback" and other signal processing, and are useful for synchronizing an oscilloscope when inspecting the television wave form at vertical field rate. Blanking is a mixture of horizontal and vertical information used to control the "blackout" of a television system during deflection return or "flyback" between horizontal lines and vertical fields. The blanking pulses are wider than drive and sync pulses in order to insure that all transitions are invisible, i.e. blanked. Sync is a complex combination of horizontal and vertical pulse information used to control the decoding of a television signal by the display device, recorder or other type of video processing equipment. Sync is usually mixed with the video information to produce a composite picture signal.
Different types of television systems, e.g. domestic broadcast, foreign broadcast, industrial and military, operate at many different scanning rates. Complete specification of the scan rate requires information as to the horizontal scanning line repetition rate (usually expressed in kHz), the number of scanning lines to be developed in each television field, the number of television fields which make up a complete frame, and the width of pulses present within the various synchronizing wave forms.
The format of H drive, V drive and blanking is the same for all scanning rates, however, the sync wave form can occur in any one of three formats e.g. broadcast, industrial and military. In the broadcast system, the vertical sync pulse interval is preceded by a front porch interval and followed by a back porch interval, each having a duration of three scanning line intervals. Six 2H rate equalizing pulses are superimposed on the sync signal during the front porch and back porch intervals, i.e. six equalizing pulses during each such interval, and six 2H rate serration pulses are superimposed on the vertical sync pulse. Domestic broadcast sync also includes horizontal front porch intervals preceding the horizontal sync pulses.
The sync for industrial television is the same as broadcast television except for the omission of the equalizing pulses and serration pulses. The sync for military television does not include equalizing and serration pulses, has no horizontal front porch, and no horizontal rate pulse information during the vertical front porch interval of three scanning lines.
The scan rate is determined by the number of scan lines per field and the desired field repetition rate. Most television systems are "interlaced" to avoid a visible flicker in the display, usually at a 2:1 ratio; 2:1 interlaced ratio means that two entire display "fields" must be developed to generate one complete "frame" or image. In the United States, field and frame rates are 60 Hz and 30 Hz respectively, whereas in countries having 50 Hz power including the European countries and Japan, the field and frame rates are 50 Hz and 25 Hz, respectively.
Television operation without interlacing, commonly referred to as 1:1 interlace, is also fairly common.
Conventional sync generator circuits for test pattern generators have typically provided only one scanning rate, depending upon the type of television system with which the test pattern generator is to be used. It is therefore desirable to provide a selectable rate sync generator in which the scanning rate may be selected over a wide range.